The original three (also called the trilogy) came into being before Area 51 existed:
- Stack Overflow
- Server Fault
- Super User
Sites fully migrated from Stack Exchange 1.0
After that, three sites that were created via the former Stack Exchange 1.0 model got fully merged into the network (the idea behind Stack Exchange 1.0 was to host sites on different topics for other people in exchange for payment - this didn't work):
- MathOverflow - sole site to be migrated "as is", with no change in scope or association to an Area 51 proposal. The site is operated by Stack Exchange on behalf of its original owners under a special agreement. Additionally, it was allowed to retain many design elements of Stack Exchange 1.0 that were later removed, until all network sites were redesigned in 2018.
- Mi Yodeya - was associated with a later Area 51 proposal for a Judaism site, but wasn't created on Area 51 per se
- Sound Design - this site was previously known as Social Sound Design with a narrower scope. Upon becoming a member of the network, the site known as Audio-Video Production had all its audio questions moved to this site, and its scope was narrowed to just Video Production.
There were some other Stack Exchange 1.0 sites that were created, but they either had their content seeded into later sites created through Area 51, or ceased to be Stack Exchange sites (with some migrating their content over to non-Stack Exchange software).
Meta sites about Stack Exchange
- Stack Apps - which, though using the same Q&A engine, is not really a Q&A site; manually created in order to host and promote programs using the Stack Exchange API
- Meta Stack Exchange - came into being as Meta Stack Overflow to supersede an old meta discussion site on the UserVoice platform before Area 51 existed; subsequently renamed and re-scoped, with a new per-site meta created for Stack Overflow
- Area 51 and Area 51 Discussions, for obvious reasons
hashcode.ru acquisition (fully migrated sites)
In 2015, Stack Exchange purchased hashcode.ru, which was a Russian operator of Stack Exchange-like Q&A sites. Some of their sites were migrated into our network, never going through Area 51:
- ru.stackoverflow.com - for programmers, in Russian (later associated with a previous Area 51 proposal for a Russian-language version of Stack Overflow, but wasn't created through Area 51)
- rus.stackexchange.com - for Russian language, in Russian (not to be confused with russian.stackexchange.com, or Russian Language in English, which was created through Area 51)
- ru.meta.stackoverflow.com - today the per-site meta for Stack Overflow in Russian; however, it was previously known as meta.hashcode.ru and served as the global meta for all sites in the hashcode.ru network (no per-site metas); content from that site was migrated in
hashcode.ru acquisition (non-migrated, separately operated sites)
In addition to the above sites which were migrated into our network, Stack Exchange runs the following sites previously operated by hashcode.ru (also known as sezn.ru), which weren't created through Area 51. These sites are entirely separate from the rest of the Stack Exchange network and operated on their own server, which nevertheless is legally owned by Stack Exchange:
- math.hashcode.ru - for mathematics help, in Russian; was large enough to be considered for migrating into our network, but community members there objected to importing the site since the rules regarding homework questions would have to be changed
- phys.sezn.ru - for physics enthusiasts, in Russian; was supposed to be shut down as part of a mass shutdown of hashcode.ru communities that were deemed too small for migration, but kept up due to oversight from the team; was decided to retain in current form due to a small number of active users
- turism.sezn.ru - for travel questions, in Russian; see above entry
There is the following site, which was created in collaboration with edX, but is not technically part of the network:
- cs50.stackexchange.com - for those completing the CS50x edX course
This category was created as a special partnership with edX, to have their forum system migrated to our engine. There were plans to create more such sites for all their courses, and a few more were created, but things just didn't work out and the partnership was dissolved and all these sites except the one above were shut down.
- ja.stackoverflow.com - this site was proposed and created through Area 51, but the proposal no longer exists. It was removed as part of a mass deletion of all non-English proposals in 2015 due to a lack of resources, but shortly afterwards, space opened up for one more site. The site was then launched, but the Area 51 proposal remains deleted.
I believe these are all of them.